In recent years, liquid crystal displays, plasma displays, and organic EL displays have been actively developed regardless of sizes thereof. An attempt has been made to use an amorphous silicon thin film, a low-temperature poly-silicon thin film, or an oxide semiconductor thin film for a thin-film transistor (TFT) serving as a pixel driving device for the displays. In particular, a TFT using an amorphous oxide semiconductor film for a channel can be applied to a flexible display using a plastic substrate formed at room temperature. Therefore, active research and development have been made as disclosed in Applied Physics Letters, Volume 89, 112123-1, pp. 1 to 3.
In the TFT disclosed in Applied Physics Letters, Volume 89, 112123-1, pp. 1 to 3, an amorphous InGaZnO4 thin film is used which is deposited as an amorphous oxide semiconductor channel layer by a sputtering method. A Y2O3 film is used which is deposited as a gate-insulator film by a sputtering method. The Y2O3 film serving as the gate-insulator film is more expensive than a silicon oxide (SiO2) film normally used as the gate-insulator film of the TFT. Therefore, the conversion of the Y2O3 film to a silicon-containing amorphous insulator film such as a SiO2 film has been expected.
The silicon-containing amorphous insulator film (hereinafter, referred to also as insulating film) such as the SiO2 film is normally produced by a chemical vapor deposition method (CVD method) in many cases. In order to obtain an insulator film having excellent insulation performance by the CVD method, a deposition temperature equal to or higher than 300° C. is normally necessary, so it is difficult to form the insulator film on a plastic substrate.
As described above, the amorphous oxide semiconductor film used for the TFT can be formed by a sputtering method at a deposition temperature equal to or lower than 300° C. Therefore, it is desirable that the insulator film can be formed by a sputtering method as in the case where the amorphous oxide semiconductor film is formed. This is because a substrate having a low heat resistance, such as the plastic substrate, can be employed.
However, the conventional silicon-containing amorphous insulator film deposited by a sputtering method is inferior in insulation characteristic to the conventional silicon-containing amorphous insulator film deposited by a CVD method. Therefore, the silicon-containing amorphous insulator film deposited by the sputtering method has not been put to practical use up to now. Thus, for example, when devices are formed on the plastic substrate, it is necessary to form the insulator film at a temperature lower than the deposition temperature using the CVD method. As described in Japanese Patent Application Laid-Open No. H02-90568, it is desired to improve the performance of the insulator film formed by the sputtering method.
In Japanese Patent Application Laid-Open No. H02-258614, an SiO2 film containing 5 atomic % or less of Ar is disclosed as a magnetic gap material of a magnetic head. However, no applications of the SiO2 film to the gate-insulator film of the TFT are disclosed.